1. Field of the Invention
This invention relates to a novel silacyclohexane compound. The invention also relates to a liquid crystal composition composing the silacyclohexane compound and to a deuce composing the composition.
2. Description of the Prior Art
The liquid crystal display devices make use of optical anisotropy and dielectric anisotrophy of liquid crystal substances. Depending on the mode of display, a variety of display systems are known including those of a twisted nematic type (TN type), a supertwisted nematic type (STN type), a super birefringence the (SBE type), a dynamic scattering type (DS type), a guest/host type, a the of deformation of aligned phase (DS type), a polymer dispersion type (PD type), and an optical mode interference type (OMI type). The most popular display device is one which is based on the Schadt-Helfrich effect and has a twisted nematic structure.
Although the properties of the liquid crystal substances used in these liquid crystal devices depend, more or less, on the the of display, it is commonly required that the liquid crystal substances have a wide range of temperatures working as a liquid crystal and that they be stable against moisture, air, light, heat, electric field and the like. Moreover, the liquid crystal substances should desirably be low in viscosity and should ensure a short address time, a low threshold voltage and a high contrast in cells.
Liquid substances which can satisfy all the requirements have never been known when used singly. In practice, several to ten and several liquid compounds and/or latent liquid crystal compounds are mixed to provide a liquid crystal mixture. To this end, it is important that constituent components be readily compatible with one another.
Among these constituent components, compounds having ring structures of phenyl cyclohexanecarboxylates of the following formulas are known as having a relatively low viscosity and capable of decreasing a threshold voltage. ##STR2## wherein R represents an alkyl group having from 1 to 9 carbon atoms as set out in Japanese Patent Publication No. 61-21937. ##STR3## wherein R represents an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 1-50691. ##STR4## wherein R represents an alkyl group from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 1-50693. ##STR5## wherein R is an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 1-50694. ##STR6## wherein R is an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 3-9895.
Moreover, known compounds whose dielectric anisotropy, .DELTA..epsilon., is negative are ones mentioned below. ##STR7## wherein R and R' are, respectively, an alkyl group having from 4 to 7 carbon atoms as set out in Japanese Patent Publication No. 60-50177. ##STR8## wherein R and R' are, respectively, an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 60-52744.
As the liquid crystal display devices recently have wider utility, the characteristic properties required for the liquid crystal materials become severer. In particular, the high performance characteristics of the materials have now been demanded owing to the requirements for high response speed and function in portableness. In order to realize the high speed response, liquid crystal compositions should have low viscosity. For the portability, limitation is inevitably placed on a power supply, so that it is necessary that the drive voltage be low or the power consumption be low, i.e. the liquid crystal composition used be low in threshold voltage. On the other hand, in multiplex drive systems wherein a high contrast is essentially required, threshold characteristics should be sharp or the dielectric constant ratio between the dielectric anisotropy, .DELTA..epsilon., and the dielectric constant along a minor axis, .epsilon..perp., should be small. Most liquid crystal compounds have a dielectric anisotropy, .DELTA..epsilon., which is a positive value or is close to zero. In order to reduce the dielectric constant ratio of a mixed composition of the compounds, liquid crystal compounds whose value of .DELTA..epsilon. is negative are necessary.